BERGENDAL, G. On spectral functions belonging to an elliptic differential operator with variable coefficients. MATHEMATICA SCANDINAVICA, [S. l.], v. 5, p. 241–254, 1957. DOI: 10.7146/math.scand.a-10501. Disponível em: https://www.mscand.dk/article/view/10501. Acesso em: 3 may. 2024.